Boros István (szerk.): A Magyar Természettudományi Múzeum évkönyve 52. (Budapest 1960)
Ravasz, Cs.: Petrographical study of the biotitic pyroxene andesite of Gyöngyöstarján
occurs a pseudoagglomerate of andésite. Further west, a relatively still quite massive and stable, pink and red endoxvandesite is encountered. Finally, on the western flank of the gorge, there occurs light green to yellowish green pseudotuff, its banking dipping towards 4 h . Towards the west, it continues in the Más Creek valley, up to the Mérőkő Creek valley. In endoxyandesite, only the limonitization of magnetite and the decomposition of the dark ingredients has commenced as yet. On the other hand, hvdroandesite was almost totally decomposed into clay minerals. Through crossed niçois it is readily seen that more than 10 per cent of the rock con sits of isotropic clay minerals, i. e. that the rock is a hydroandesite indeed. The andesitic origin of the rock is proven by the still recognizable microholocrystalline texture on the one hand and by the mineralogical composition of the porphyries, on the other. In some of the feldspars, the suteres of polylamellar twinning are still visible. The decomposed dark ingredients have shapes suggestive of pyroxenes, and show the characteristic cleavage pattern. In the pseudotuff, the original texture of rocks can be seen in larger-size hand specimens only. A more favorable disclosure of the pseudotuff occurs in the Más Creek valley, west of the cattle well. On the lower level of the rock wall of about 12 metres height, the traces of gobular alteration are still visible, while on the upper level the rock is crumbling powdery. The pseudotuff is light green, with a banking oriented 5 h /15°. The microscopic properties of a sample from this disclosure areas follows : In the less altered specks, metavolcanitic opacitization is predominating, with abundant fresh feldspar needles and laths. The porphyric ingredients of the more intensely altered parties are carbonatized, the feldpars and most of the dark ingredients being replaced by calcite. The matrix is transformed into a mass of clay minerals, containing no visible magnetite and limonite. Of the feldspar laths, the ones smaller than 5 [i were preserved only, and even these only infrequently. No slide can be prepared of the pseudotuff : its matrix is thoroughly friable, containing angular grains of rock of 1 to 5 millimetres diameter, being in the oxyandesite stage. Under a magnifying lens, no mineralogical component at all is recognizable in the matrix. The endometavolcanization of biotitic pyroxene andésite can be observed in the Más Creek valley. In the immediate neighbourhood of the occurrence, the orientation of the banking becomes confuse. The dip towards 4 to 6 h , prevailing farther away, changes to 20—16—ll h towards the north, then to 6 h again, with the corresponding dip angles being 10—5—0—25°, and finally 45°. A similar confusion is observed on the other side of the creek. All this suggests that the alteration of the andésite was due to tectonical causes. The circulating solutions have effected an oxyandesitization along the gaping fissures, while farther towards the interior of the rock block a hydroandesite mantle was formed. The microscopical picture presented by an endoxyandesite formed by the alteration of biotitic pyroxene andésite does not differ significantly from that of typical biotitic pyroxeneandesite. Under the microscope, the alkali-leaching effect of the endometavolcanization process is easily recognized. Of the biotites, no more than the marginal parts were preserved in most cases. In the basic medium having come to exist in this way, the entire rock has become infiltrated by limonite, apart from the characteristic metavolcanitic opacitization. Of the feldspars, frames of 20 to 50 fx width have remained ; their interiors have changed into chlorite and sericite. The matrix contains much glass and a great number of feldspar laths smaller than 5
